Heretofore, in a photolithographic technology, an exposure apparatus has been widely utilized which transfers a fine circuit pattern on a wafer to produce an integrated circuit. Along with high integration and high functionality of an integrated circuit, an integrated circuit becomes finer, and an exposure apparatus is required to form an image of a circuit pattern with high resolution on a wafer with a deep focal depth, and shortening of the wavelength of the exposure light source is being advanced. The exposure light source has been advanced from conventional g-line (wavelength: 436 nm) or i-line (wavelength: 365 nm), and now, a KrF excimer laser (wavelength: 248 m) or an ArF excimer laser (wavelength: 193 nm) is being used.
For an optical member for an exposure apparatus employing a light having a wavelength of from 180 to 400 nm as a light source, a synthetic quartz glass has been mainly employed, since it is excellent in transparency over a wide range from a near infrared region to an ultraviolet region, it has an extremely small thermal expansion coefficient and it is relatively easily processed. As a conventionally employed synthetic quartz glass, one as disclosed in JP-A-3-88742 may, for example, be known. That is, a synthetic quartz glass having a OH group content of at least 10 ppm and containing hydrogen in an amount of at least 5×1016 molecules/cm3 has been known.
When a synthetic quartz glass is irradiated with ultraviolet lights, defect precursors such as distorted three-membered ring structures or four-membered ring structures are dissociated to form paramagnetic defects such as an E′ center (≡Si.) and NBOHC (≡SiO.). Such paramagnetic defects have absorption bands centered at a wavelength of 220 nm and at a wavelength of 260 nm, respectively, and thus the formation of the paramagnetic defects is a main cause of the decrease in the transmittance. Hydrogen molecules in the synthetic quartz glass are considered to have a function to convert the E′ center and the NBOHC into ≡SiH and ≡SiOH having no absorption band centered at a wavelength of from 190 to 400 nm, respectively, and the above JP-A-3-88742 is a proposal which pays attention to this defect repairing effect of hydrogen molecules.
However, ≡SiH and ≡SiOH are dissociated by irradiation with ultraviolet lights and converted into the E′ center and the NBOHC again, and accordingly the defect formation process during irradiation with ultraviolet lights is complicated, and it is considered that the following four processes are in progress.

If the irradiation of the synthetic quartz glass with ultraviolet lights is intermitted, only the process of the above formula (2) proceeds, the E′ center and the NBOHC which are present during the irradiation are sequentially converted to SiH and SiOH structures, and the synthetic quartz glass gradually recovers from the transmittance decreased state during the irradiation with ultraviolet lights. However, if the irradiation with ultraviolet lights is carried out again, two processes of the above formulae (3) and (4) immediately proceed, and the transmittance immediately decreases to the level immediately before intermission of the irradiation.
Here, in practical application to e.g. an optical member for a semiconductor exposure machine, as shown in FIG. 2, the irradiation with ultraviolet lights is carried out usually intermittently (this irradiation method is referred to as “burst mode”) for alignment, wafer exchange, job change etc.
In such a case, the change in the transmittance decreases the control accuracy of the exposure amount and leads to deterioration in the pattern dimensional accuracy, and accordingly the change in the transmittance when the irradiation with ultraviolet lights is carried out intermittently is preferably as small as possible.
Particularly, the exposure amount of a semiconductor exposure machine is controlled usually by an illuminometer located between a projection lens and a light source. The change in the transmittance of the projection lens itself remarkably deteriorates the control accuracy of the exposure amount, and accordingly it is particularly desirable to employ a glass material with a small change in the transmittance. The above JP-A-3-88742 is a proposal intended to suppress the decrease in the transmittance when continuous irradiation with ultraviolet lights is carried out, simply by incorporating hydrogen molecules at a certain concentration or above, and the change in the transmittance in a case of irradiation under irradiation conditions close to the practical use conditions, i.e. by the burst mode, is not considered, and the synthetic quartz glass by the above proposal has a problem with the amount of the change in the transmittance in a case of irradiation with ultraviolet lights by the burst mode in some cases.
It is an object of the present invention to provide a synthetic quartz glass with a small decrease in the transmittance by irradiation with ultraviolet lights and with a small amount of the change in the transmittance when irradiated with ultraviolet lights intermittently.